Stt-mram cell structures

ABSTRACT

A magnetic cell structure including a nonmagnetic bridge, and methods of fabricating the structure are provided. The magnetic cell structure includes a free layer, a pinned layer, and a nonmagnetic bridge electrically connecting the free layer and the pinned layer. The shape and/or configuration of the nonmagnetic bridge directs a programming current through the magnetic cell structure such that the cross sectional area of the programming current in the free layer of the structure is less than the cross section of the structure. The decrease in the cross sectional area of the programming current in the free layer enables a lower programming current to reach a critical switching current density in the free layer and switch the magnetization of the free layer, programming the magnetic cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/595,955, which was filed on Jan. 13, 2015, which is a divisional ofU.S. patent application Ser. No. 14/037,064, which was filed on Sep. 25,2013, now U.S. Pat. No. 8,945,950, which issued on Feb. 3, 2015, whichis a divisional of U.S. patent application Ser. No. 12/351,517, whichwas filed on Jan. 9, 2009, now U.S. Pat. No. 8,553,449, which issued onOct. 8, 2013.

BACKGROUND

Field of Invention

The invention relates generally to magnetic random access memory, andmore particularly, to Spin Torque Transfer Magnetic Random Access Memory(STT-MRAM).

Description of Related Art

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light and not as admissions of prior art.

Magnetic Random Access Memory (MRAM) is a non-volatile computer memorytechnology based on magnetoresistance. MRAIVI differs from volatileRandom Access Memory (RAM) in several respects. Because MRAIVI isnon-volatile, MRAM can maintain memory content when the memory device isnot powered. Though non-volatile RAM is typically slower than volatileRAM, MRAM has read and write response times that are comparable to thatof volatile RAM. Unlike typical RAM technologies which store data aselectric charge, MRAIVI data is stored by magnetoresistive elements.Generally, the magnetoresistive elements are made from two magneticlayers, each of which holds a magnetization. The magnetization of onelayer (the “pinned layer”) is fixed in its magnetic orientation, and themagnetization of the other layer (the “free layer”) can be changed by anexternal magnetic field generated by a programming current. Thus, themagnetic field of the programming current can cause the magneticorientations of the two magnetic layers to be either parallel, giving alower electrical resistance across the layers (“0” state), orantiparallel, giving a higher electrical resistance across the layers(“1” state). The switching of the magnetic orientation of the free layerand the resulting high or low resistance states across the magneticlayers provide for the write and read operations of the typical MRAMcell.

Though MRAM technology offers non-volatility and faster response times,the MRAM cell is limited in scalability and susceptible to writedisturbances. The programming current employed to switch between highand low resistance states across the MRAM magnetic layers is typicallyhigh. Thus, when multiple cells are arranged in an MRAM array, theprogramming current directed to one memory cell may induce a fieldchange in the free layer of an adjacent cell. This potential for writesdisturbances, also known as the “half-select problem,” can be addressedusing a spin torque transfer technique.

A conventional spin torque transfer MRAM (STT-MRAM) cell may include amagnetic cell stack, which may be a magnetic tunnel junction (MTJ) or aspin valve structure. An MTJ is a magnetoresistive data storing elementincluding two magnetic layers (one pinned and one free) and aninsulating layer in between, a bit line, a word line, a source line, andan access transistor. A spin valve has a structure similar to the MTJ,except a spin valve has a conductive layer in between the two magneticlayers. A programming current typically flows through the accesstransistor and the magnetic cell stack. The pinned layer polarizes theelectron spin of the programming current, and torque is created as thespin-polarized current passes through the stack. The spin-polarizedelectron current interacts with the free layer by exerting a torque onthe free layer. When the torque of the spin-polarized electron currentpassing through the stack is greater than the critical switching currentdensity (J_(c)), the torque exerted by the spin-polarized electroncurrent is sufficient to switch the magnetization of the free layer.Thus, the magnetization of the free layer can be aligned to be eitherparallel or antiparallel to the pinned layer, and the resistance stateacross the stack is changed.

The STT-MRAM has advantageous characteristics over the MRAM, because thespin-polarized electron current eliminates the need for an externalmagnetic field to switch the free layer in the magnetoresistiveelements. Further, scalability is improved as the programming currentdecreases with decreasing cell sizes, and the writing disturbance andhalf-select problem is addressed. Additionally, STT-MRAM technologyallows for a higher tunnel magnetic resistance ratio, meaning there is alarger ratio between high and low resistance states, improving readoperations in the magnetic domain.

However, high programming current densities through the STT-MRAM cellmay still be problematic. High current densities through the magneticlayers may increase the energy consumption in the cell and the thermalprofile in the layers, affecting the cell's integrity and reliability,and may also lead to larger silicon real estate consumption for eachcell.

BRIEF DESCRIPTION OF DRAWINGS

Certain embodiments are described in the following detailed descriptionand in reference to the drawings in which:

FIG. 1 depicts a block diagram of a processor-based system in accordancewith an embodiment of the present technique;

FIG. 2 depicts a schematic diagram of a portion of a memory array havingmemory cells fabricated in accordance with embodiments of the presentinvention;

FIG. 3A depicts a magnetic cell structure with a nonmagnetic layerconnecting a free layer and a pinned layer in accordance withembodiments of the present invention;

FIG. 3B depicts an example of a current path through the magnetic cellstructure of FIG. 3A.

FIGS. 4A and 4B depict side views and top views of fabricating themagnetic cell structure depicted in FIGS. 3A and 3B;

FIGS. 5A and 5B depict side views and top views of fabricating amagnetic cell structure with nonmagnetic material connecting a freelayer and a pinned layer in accordance with embodiments of the presentinvention;

FIGS. 6A and 6B depict side views and top views of fabricating anothermagnetic cell structure with nonmagnetic material connecting a freelayer and a pinned layer in accordance with embodiments of the presentinvention;

FIGS. 7A and 7B depict side views and top views of one method offabricating yet another magnetic cell structure with nonmagneticmaterial connecting a free layer and a pinned layer in accordance withembodiments of the present invention; and

FIGS. 8A and 8B depict side views and top views of another method offabricating the magnetic cell structure fabricated in FIGS. 7A and 7B inaccordance with embodiments of the present invention.

DETAILED DESCRIPTION

As previously discussed, a spin torque transfer magnetic random accessmemory (STT-MRAM) cell is programmed by switching the magnetization ofthe free layer in the cell's magnetic cell stack. Switching occurs whenthe current density passing through the memory cell is larger than thecritical switching current density. Thus, to program the cell, theprogramming current density need only be slightly higher than thecritical switching current density. Since passing a larger programmingcurrent increases the energy consumption and the thermal profile in thecell stack, which affects the integrity and reliability of the cell, itis desirable to decrease the critical switching current withoutaffecting the cell's thermal stability. Applying a lower programmingcurrent while maintaining a programming current density that is abovethe critical switching current density would allow a smaller current toswitch the free layer of the cell. The following discussion describesthe systems and devices, and the operation of such systems and devicesin accordance with the embodiments of the present technique.

FIG. 1 depicts a processor-based system, generally designated byreference numeral 10. As is explained below, the system 10 may includevarious electronic devices manufactured in accordance with embodimentsof the present technique. The system 10 may be any of a variety of typessuch as a computer, pager, cellular phone, personal organizer, controlcircuit, etc. In a typical processor-based system, one or moreprocessors 12, such as a microprocessor, control the processing ofsystem functions and requests in the system 10. As is explained below,the processor 12 and other subcomponents of the system 10 may includeresistive memory devices manufactured in accordance with embodiments ofthe present technique.

The system 10 typically includes a power supply 14. For instance, if thesystem 10 is a portable system, the power supply 14 may advantageouslyinclude a fuel cell, a power scavenging device, permanent batteries,replaceable batteries, and/or rechargeable batteries. The power supply14 may also include an AC adapter, so the system 10 may be plugged intoa wall outlet, for instance. The power supply 14 may also include a DCadapter such that the system 10 may be plugged into a vehicle cigarettelighter, for instance.

Various other devices may be coupled to the processor 12 depending onthe functions that the system 10 performs. For instance, a userinterface 16 may be coupled to the processor 12. The user interface 16may include buttons, switches, a keyboard, a light pen, a mouse, adigitizer and stylus, and/or a voice recognition system, for instance. Adisplay 18 may also be coupled to the processor 12. The display 18 mayinclude an LCD, an SED display, a CRT display, a DLP display, a plasmadisplay, an OLED display, LEDs, and/or an audio display, for example.Furthermore, an RF sub-system/baseband processor 20 may also be coupledto the processor 12. The RF sub-system/baseband processor 20 may includean antenna that is coupled to an RF receiver and to an RF transmitter(not shown). One or more communication ports 22 may also be coupled tothe processor 12. The communication port 22 may be adapted to be coupledto one or more peripheral devices 24 such as a modem, a printer, acomputer, or to a network, such as a local area network, remote areanetwork, intranet, or the Internet, for instance.

The processor 12 generally controls the system 10 by implementingsoftware programs stored in the memory. The software programs mayinclude an operating system, database software, drafting software, wordprocessing software, and/or video, photo, or sound editing software, forexample. The memory is operably coupled to the processor 12 to store andfacilitate execution of various programs. For instance, the processor 12may be coupled to the system memory 26, which may include spin torquetransfer magnetic random access memory (STT-MRAM), magnetic randomaccess memory (MRAM), dynamic random access memory (DRAM), and/or staticrandom access memory (SRAM). The system memory 26 may include volatilememory, non-volatile memory, or a combination thereof. The system memory26 is typically large so that it can store dynamically loadedapplications and data. In some embodiments, the system memory 26 mayinclude STT-MRAM devices, such as those discussed further below.

The processor 12 may also be coupled to non-volatile memory 28, which isnot to suggest that system memory 26 is necessarily volatile. Thenon-volatile memory 28 may include STT-MRAM, MRAM, read-only memory(ROM), such as an EPROM, resistive read-only memory (RROM), and/or flashmemory to be used in conjunction with the system memory 26. The size ofthe ROM is typically selected to be just large enough to store anynecessary operating system, application programs, and fixed data.Additionally, the non-volatile memory 28 may include a high capacitymemory such as a tape or disk drive memory, such as a hybrid-driveincluding resistive memory or other types of non-volatile solid-statememory, for instance. As is explained in greater detail below, thenon-volatile memory 28 may include STT-MRAM devices manufactured inaccordance with embodiments of the present technique.

FIG. 2 illustrates an STT-MRAIVI cell 50, which may be fabricated toform an array of memory cells in a grid pattern including a number ofrows and columns, or in various other arrangements depending on thesystem requirements and fabrication technology. An arrangement of memorycells may be implemented in the system memory 26 or the volatile memory28 depicted in FIG. 1.

The STT-MRAM cell 50 includes a magnetic cell structure 52, an accesstransistor 54, a bit line 56, a word line 58, a source line 60,read/write circuitry 62, a bit line reference 64, and a sense amplifier66. The magnetic cell structure 52 may include a spin valve. As will bedescribed further below with specific reference to FIGS. 3A-8B, thestructure 52 may further include a nonmagnetic bridge between the freeand pinned layers in accordance with embodiments of the presenttechnique. In various embodiments described below, the nonmagneticbridge in the structure 52 may be layers disposed within or around thestructure 52, or layered along the structure 52 (FIGS. 3A-8B).

As used herein, the STT-MRAIVI cell 50 generally includes a “magneticcell structure.” The magnetic cell structure may be a spin valve, asdiscussed above, if a nonmagnetic conductive material is between a freelayer and a pinned layer. As used in the present specification, the term“structure” may include a magnetic cell structure, and may refer to amemory cell structure, magnetic cell structure, STT-MRAM cell structure,or any component of a memory cell which may include layers and materialsin accordance with an embodiment of the present technique. Furthermore,the term “structure” may refer to transitional structures duringprocesses to fabricate the magnetic cell structure of the presenttechniques.

As will be explained, the term “bridge” may refer to a layer, a liner, aspacer, a strip, or some other formation which provides a path orelectrical connection between the free and pinned layers. The bridge maybe parallel or perpendicular to the structure (i.e., the layers of thestructure), and may be formed by deposition, growth, or any otherprocess(es) in accordance with embodiments of the present technique. Asalso used herein, materials may be referred to as a “layer” when thematerial is formed above or below the structure or within the structure(e.g., a liner or a strip). A layer may be either parallel orperpendicular to the structure. It should be understood that when alayer is said to be “formed on,” “formed below,” “disposed on,” or“disposed below” another layer, there may be intervening layers formedor disposed between those layers. Conversely, if a layer or material issaid to be “formed directly on,” “formed directly below,” “disposeddirectly on,” “disposed directly below,” or “in direct contact with,”the materials or layers include no intervening materials or layerstherebetween.

When the STT-MRAM cell 50 is selected to be programmed, a programmingcurrent is applied to the cell, and the electrons of the current arespin-polarized by the pinned layer to exert a torque on the free layer,which switches the magnetization of the free layer to “write to” or“program” the cell. To initiate the write operation, the read/writecircuitry 62 may generate a write current to the bit line 56 and thesource line 60. The polarity of the voltage between the bit line 56 andthe source line 60 determines the switch in magnetization of the freelayer in the structure 52. Furthermore, and as discussed in detailbelow, incorporating a nonmagnetic material to electrically connect thefree and pinned layers may direct a programming current flow through themagnetic cell structure 52 to decrease the cross sectional area of theprogramming current flow. By decreasing the cross sectional area of theprogramming current through the free layer, a smaller programmingcurrent may still result in a programming current density in the freelayer that is greater than the critical switching current densityrequired to switch the magnetization of the free layer. Thus, a smallerprogramming current may write the STT-MRAM cell 50. Once the free layeris magnetized according to the spin polarity of the programming current,the programmed state is written to the STT-MRAM cell 50.

In a read operation of the STT-MRAM cell 50, a current is used to detectthe resistance state of the magnetic cell structure 52. To initiate aread operation, the read/write circuitry 62 generates a read current tothe bit line 56 and the source line 60 through the structure 52 and thetransistor 54. The programmed state of the STT-MRAM cell 50 depends onthe resistance across the structure 52 which may be determined by thevoltage difference between the bit line 56 and the source line 60. Insome embodiments, the voltage difference may be compared to a reference64 and amplified by a sense amplifier 66.

One embodiment of the present techniques for programming a STT-MRAM cellwith a decreased programming current, is illustrated in FIGS. 3A and 3B.The STT-MRAM cell includes a memory cell structure 100 having anonmagnetic layer 114 which may be employed to decrease the crosssectional area of the programming current flow. As will be furtherexplained, this technique enables a lower programming current tofacilitate a switch in the magnetization of the free layer 106 toprogram the memory cell.

The memory cell structure 100 of FIG. 3A may include a free layer 106and a pinned layer 110 with a dielectric layer 108 in between. Thepinned layer 110 is so named because it has a magnetization with a fixedor preferred orientation, and this is represented by the markingindicating that the magnetization of the pinned layer 110 is in adirection into the page. The free layer 106 has a magnetization whichmay be switched to allow the memory cell to be programmed. As previouslydiscussed, switching the magnetization of the free layer 106 changes theresistance across the free layer 106 and the pinned layer 110 such thatthe memory cell is programmed to either a low resistance state (when thefree layer 106 and the pinned layer 110 have parallel magnetizations) ora high resistance state (when the free layer 106 and the pinned layer110 have antiparallel magnetizations). The cell may be read bydetermining the resistance across the free layer 106 and the pinnedlayer 110.

Switching the magnetization of the free layer 106 occurs when aprogramming current passed through the memory cell has a current densitythat is greater than a critical switching current density. Typically, aprogramming current is applied perpendicularly through the layers of anSTT-MRAm cell structure. Since the programming current is flowingaxially through the free layer, the programming current density in thefree layer would typically be the electric current per cross sectionalarea, or the electric current in amperes, divided by the width and depthof the free layer. However, as discussed below in accordance withembodiments of the present invention, the programming current can bereduced without departure on the volume of the free layer

In one embodiment, a nonmagnetic layer 114 is formed between the freelayer 106 and the pinned layer 110. When the cell is selected to beprogrammed, the programming current may flow through the pinned layer110 and the nonmagnetic layer 114, and laterally through the free layer106. The nonmagnetic layer 114 provides electrical connectivity betweena side of the pinned layer 110 and a side of the free layer 106.Further, the dielectric layer 108 may provide insulation between thepinned layer 110 and the free layer 106, such that the programmingcurrent will flow through the nonmagnetic layer 114 and laterallythrough the free layer 106, rather than axially through the free layer106. Since the cross sectional area of the current flow through the freelayer 106 would be significantly greater in the axial direction than inthe lateral direction, a smaller programming current flowing laterallymay have the same current density in the free layer 106 as acomparatively larger programming current flowing axially through thefree layer 106.

Referring to the current density relationship previously described, thecurrent density through the free layer 106 when the programming currentflows laterally, may be the electric current in amperes, divided by theheight and depth of the free layer 106. In some embodiments, the height(or thickness) of the free layer 106 may be smaller than the width ofthe free layer 106. Consequently, a much smaller programming current mayhave a sufficient current density when flowing through the free layer106 to write the STT-MRAM cell. As used herein, the depth may refer tothe length of a layer in a direction into the page, and the width mayrefer to the length of a layer across the page, as may be seen in theside views of the magnetic cell structures in FIGS. 3A, 4A, 5A, 6A, 7A,and 8A. The height may refer to the top to bottom length of a layer. Theheight may also refer to a thickness of a layer.

FIG. 3B depicts the STT-MRAM cell structure 100 as in FIG. 3A with anexample of the programming current flow in accordance with embodimentsof the present techniques. The arrows in the structure 100 represent thedirection of current flow. The programming current may flow laterallythrough the pinned layer 110 in a direction to the left, up thenonmagnetic layer 114, and laterally through the free layer 106 in adirection to the right, and then up to the electrode 102. As discussed,the pinned layer 110 may be magnetized to a direction into the page, andas the programming current is flowing laterally to the right through thefree layer 106, the spin torque applied to the free layer 106 maymagnetize it to a direction out of the page, as indicated by the markingin the free layer 106. Thus, in this example, the pinned layer 110 andthe free layer 106 have an antiparallel magnetization, and the STT-MRAMcell has been programmed to a high resistance state.

The structure 100 may also include an antiferromagnetic layer 112 belowthe pinned layer 110 to achieve the pinning through exchange couplingand further increase cell stability. The structure may also include anelectrode 102 making contact to the free layer 106 and anotherdielectric layer 104 on top of the free layer 106. This embodiment, andeach of the embodiments illustrated and described below, may beincorporated into the STT-MRAM cell 50, and further incorporated into anSTT-MRAM cell array, as described in FIG. 2.

The examples of materials discussed below may be used in an embodimentas in FIGS. 3A-3B, or in any other embodiment in accordance with thepresent technique. In some embodiments, the free layer 106 and thepinned layer 110 may comprise ferromagnetic materials, such as Co, Fe,Ni or its alloys, NiFe, CoFe, CoNiFe, or doped alloys CoX, CoFeX,CoNiFeX (X═B, Cu, Re, Ru, Rh, Hf, Pd, Pt, C), or other half-metallicferromagnetic material such as Fe3O4, CrO2, NiMnSb and PtMnSb, andBiFeO, for instance. The nonmagnetic layer 114 connecting the free layer106 and the pinned layer 110 may comprise materials such as Cu, Au, Ta,Ag, CuPt, CuMn, any combination of the above materials, or any otherconductive nonmagnetic materials. The dielectric layers 108 and 104 maycomprise any insulative material, such as SiN, and may have a thicknessof approximately 10 nm-30 nm.

FIG. 4A depicts side views of magnetic cell structures 120, 130, 140,150, and 160 along different steps in a fabrication process to form theSTT-MRAM cell structure 100 depicted in FIGS. 3A and 3B. FIG. 4B depictstop views of two structures 120, 130, 140, 150, and 160 at the differentfabrication steps corresponding to the steps in FIG. 4A, and inaccordance with an embodiment of the present invention. While the sideviews depicted in FIG. 4A show one structure 120, 130, 140, 150, and 160at each step, the top views in FIG. 4B may show two structures 120, 130,140, 150, and 160 as an example of how more than one structure may befabricated or arranged in a memory cell array.

In the first step of the fabrication process, a structure 120 comprisesa free layer 106 and a pinned layer 110 with a dielectric layer 108 inbetween. The structure 120 may also have a dielectric layer 104 disposedon the free layer 106 and an antiferromagnetic layer 112 beneath thepinned layer 110. The structure 120 is etched into stripes, stopping inthe pinned layer 110. As can be seen from the top view (structure 120FIG. 4B), the dielectric layer 104 and the pinned layer 110 may bevisible, as the structure 120 has been etched down to expose the pinnedlayer 110. After etching into the pinned layer 110, a nonmagnetic layer114 may be formed to bridge the free and pinned layer 106 and 110, asseen in the structure 130. The nonmagnetic layer 114 may be formedperpendicular to the free and pinned layers 106 and 110 and may beformed across the insulative dielectric layer 108, forming an electricalpath from the pinned layer 110 through the nonmagnetic layer 114 to thefree layer 106. As discussed, this electrical path may allow aprogramming current to flow laterally through the free layer 106, thusenabling a lower programming current to meet the critical switchingcurrent density through the free layer 106. The side view of thestructure 130 (FIG. 4A) depicts an example of such a perpendiculararrangement, and the top view of the structure 130 (FIG. 4B) may depicttwo structures 130 back-to-back.

After the nonmagnetic layer 114 is formed, the structure 140 may beetched to form a trench 142 stopping at the dielectric layer 108. As canbe seen from the top view of the structure 140, the dielectric layer 108may be visible after the trench 142 is etched. In the next structure 150of the fabrication process, an electrode 102 may then be formed in thetrench 142 (as in the previously discussed structure 140), and oneportion 154 of the electrode may make contact to a side of the freelayer 106 while another portion 152 may extend laterally over thedielectric layer 104. This configuration may allow the programmingcurrent to flow laterally through the free layer 106 from thenonmagnetic layer 114 at one edge of the free layer 106 to the portion154 of the electrode 102 at the other edge of the free layer 106. Thecorresponding top view (FIG. 4B) may depict two structures 150back-to-back. After the formation of the electrode 102, the cell may bepatterned to reduce the cell size and to isolate adjacent cells. As seenin the top view of the patterned structure 160, the cell depth may bedecreased when compared with the previously discussed structure 150.

As used herein, “etching” may refer to a chemical removal of layers inthe magnetic cell structures (or transitional structures in fabricatingthe magnetic cell structures). While different etching processes may bediscussed (e.g., isotropic etching, dry etching, trench etching, viaetching, mesa etching, etc.), any etching procedure, or any proceduresuitable for removing layers in the structures may be used in accordancewith the present techniques. For example, etching may remove a portionof a dielectric layer to expose one or more of a pinned layer or a freelayer. Also as used herein, “forming” may refer to any deposition orgrowth of a material in the structure. For example, a physical vapordeposition, chemical vapor deposition, conformal chemical vapordeposition, electroplating, or any other suitable deposition may be usedto deposit materials, layers, or spacers in the structure. Further,“patterning” may refer to any removal of parts of the structure, usingphotolithographic techniques, for example. Patterning may be used in thecurrent techniques to reduce the size of a structure or to isolate thestructure from adjacent structures in a memory array.

Although the steps described herein may be described sequentially ornumbered (e.g., first, second, etc.), the steps may not necessarily beperformed in the same order as the sequence or numbering described. Somesteps may be performed in a different order, modified, or skipped, oradditional steps may be added in accordance with the present techniques.

Another embodiment of the present techniques for reducing theprogramming current required to switch the free layer of an STT-MRAIVIcell, may be depicted in the fabrication steps of FIGS. 5A and 5B. FIG.5A may depict side views of structures 200, 210, 220, and 230 atdifferent steps in fabrication, and FIG. 5B may depict top views ofstructures 200, 210, 220, and 230 at the different fabrication stepscorresponding to FIG. 5A. The structures 200, 210, 220, and 230 in bothFIGS. 5A and 5B may depict two adjacent magnetic cell structures 200,210, 220, and 230 to give an example of how the magnetic cell structureor an array of structures may be fabricated in accordance withembodiments of the present techniques. The first step may involveetching a trench 202 into the dielectric material 108 of the structure200. The trench etching may stop at the pinned layer 110, and the topview of the structure 200 may depict that the dielectric material 108and the pinned layer 110 are visible because the trench 202 has beenetched, stopping at the pinned layer 110.

The next structure 210 depicts that a nonmagnetic liner 114 may beformed in the trench 202 along the etched side of the dielectric layer108. The nonmagnetic liner 114 may be perpendicular to the pinned layer110. After the nonmagnetic liner 114 is formed, the remaining portion ofthe trench 202 may be filled with dielectric material 108. As seen inthe structure 220, the nonmagnetic liner 114 may be perpendicular to thepinned layer 110 and surrounded on two sides by dielectric material 108.After the dielectric material 108 is filled in the trench 202, thestructure 220 may undergo chemical mechanical planarization (CMP), whichmay expose the top edge of the nonmagnetic liner 114. A free layer 106may be deposited on top of the dielectric material 108 and may be incontact with the exposed top edge of the nonmagnetic liner 114. Anelectrode 102 may also be deposited on the free layer 106, as seen inthe structure 230. As can be seen in the top view of the structure 230in FIG. 5B, the structure 230 may be patterned and isolated fromadjacent structures 230.

The structure 230 may also be achieved by other fabrication processes.For example, rather than trench etching into the dielectric material108, and then refilling the dielectric material 108 after the formationof the nonmagnetic liner 114, the dielectric material 108 may be viaetched, and nonmagnetic material may be deposited in the spaces to formnonmagnetic liners 114.

In this embodiment, the programming current may flow from the pinnedlayer 110 through the nonmagnetic liner 114 to the free layer 106. Sincethe flow of the programming current is limited to the thickness of thenonmagnetic liner 114 prior to flowing through the free layer 106, thecross sectional area of the programming current flow through the freelayer 106 may be substantially determined by the thickness of thenonmagnetic liner 114 and the depth of the free layer 106. In thisembodiment, though the programming current is still flowing in adirection axial to the free layer 106, the cross sectional area of thecurrent may be less than typical magnetic cell configurations. Intypical magnetic cells, the cross sectional area of the programmingcurrent through the free layer may be determined by the entire crosssection of the free layer in the axial direction (i.e., the width timesthe depth of the free layer). In the present structure 230, the crosssectional area of the programming current through the free layer 106 maybe significantly smaller because the current flows to the free layer 106from the nonmagnetic liner 114, which has a much smaller cross section(i.e., the width of the nonmagnetic liner 114 times the depth of thefree layer 106). Because of the smaller cross sectional area ofprogramming current through the free layer 106, the programming currentdensity in the free layer 106 may also be higher, and so a lowerprogramming current may switch the magnetization of the free layer 106or a portion of the free layer 106. In some embodiments, the switch ofmagnetization in a portion of the free layer 106 may also propagatethrough the rest of the free layer 106 after some period of time.

Switching the magnetization of the free layer 106, or some portion ofthe free layer 106 may occur when the programming current reaches acritical switching current density in the free layer 106 where the spintorque of the electrons in the programming current may switch themagnetization of the free layer 106 to write the cell to a high or lowresistance state. Using the structure 230 in FIG. 5A as an example, ifthe programming current was flowing upwards in the free layer 106, thecurrent density may be sufficient to switch the magnetization of someportion of the free layer 106 because the smaller cross sectional areaof the programming current flow through the portion of the free layer106. The spin torque of the downward flowing electrons would switch themagnetization in the free layer 106 to the left, such that the pinnedlayer 110 and the free layer 106 are antiparallel, and the structure 230is programmed to a high resistance state.

Read operations may also involve sending a read current, which may flowthrough the cell to determine the resistance between the free layer 106and the pinned layer 110. As the read current may also flow through thenonmagnetic liner 114 or some other nonmagnetic conductive material inother embodiments in accordance with the present techniques, the path ofthe read current through the free layer 106 may also be limited to anarea that is approximately the width of the nonmagnetic liner 114 andthe depth of the free layer 106. Therefore, the read current may measurethe resistance of the structure 230 through the portion of the freelayer 106 that has been switched by the programming current.

FIGS. 6A and 6B may depict another embodiment of the present techniqueswhere a nonmagnetic layer 114 may be formed on the side of a dielectriclayer 108 in a magnetic cell structure to electrically connect the freeand pinned layers 106 and 110. FIG. 6A may depict side views ofstructures 300, 310, 320, and 330 at different steps in fabrication, andFIG. 6B may depict top views of structures 300, 310, 320, and 330 at thedifferent fabrication steps corresponding to FIG. 6A.

A magnetic cell structure 300 may be first isolated into individualcells by etching. The individual structure 300 may be in an oval shape,as depicted in the structure 300 of FIG. 6B, or in any other shape inaccordance with the present techniques. Each structure 300 may include afree layer 106 and a pinned layer 110 with a dielectric layer 108 inbetween. An electrode 102 may be disposed on the free layer 106, and anantiferromagnetic layer 112 may be beneath the pinned layer 110.

After the structure 300 is etched into individual structures 300, arecess 312 may be formed in the dielectric layer 108, under the freelayer 106. For example, an isotropic etch procedure may be used to formthis recess 312. As the recess 312 is below the free layer 106 and thetop electrode 102, the recess 312 may not be seen from the top view ofthe structure 310, but an example of where the recess 312 may be locatedbeneath the top electrode 102 is depicted by the outlined recessedportion 314 in the structure 310 of FIG. 6B.

Conductive nonmagnetic material 322 may be deposited such that itsurrounds the surfaces of the structure 320, including the recess 312(as in structure 310). The deposition may be by conformal chemical vapordeposition (CVD), for example, or by any other method in accordance withthe present techniques. A top view of the structure 320 may show thatthe entire structure 320 is covered by the conductive nonmagneticmaterial 322. Unwanted portions of the nonmagnetic material 322 may beremoved by isotropic etching to produce a structure 330 with a thinnonmagnetic layer 114 remaining in the recess 312 (as in the previousstructure 310). A top view of the structure 330 may only show the topelectrode 102 of the structure 330.

In this embodiment, the cross sectional area of the programming currentthrough the free layer 106 may be limited by the small cross sectionalarea of the current path immediately before entering the free layer 106.More specifically, the programming current flows from the pinned layer110 through the nonmagnetic layer 114 before reaching the free layer106. As the cross sectional area of the programming current is small(i.e., limited to the cross sectional area of the nonmagnetic layer 114)immediately before reaching the free layer 106, the cross sectional areaof the programming current may also be small in the free layer 106. Thecross sectional area of the programming current through the free layermay be approximated by the circumference of the structure 330 and thethickness of the nonmagnetic layer 114. Thus, a smaller programmingcurrent may have a larger current density in the portion of the freelayer 106 through which the current travels than if the programmingcurrent were traveling axially through the entire cross sectional areaof the free layer 106.

FIGS. 7A-8B depict another embodiment of the present techniques, where anonmagnetic layer 114 is formed within a magnetic cell structure toelectrically connect the free layer 106 and the pinned layer 110. FIGS.7A and 7B may depict side views and top views of structures 400, 410,420, 430, and 440 in different steps of one fabrication method forcreating this embodiment, and FIGS. 8A and 8B may depict side views andtop views of structures 450, 460, 470, 480, and 490 of anotherfabrication method for creating this embodiment.

Referring first to FIGS. 7A and 7B, the structure 400 may include adielectric layer 108 on top of a pinned layer 110. An antiferromagneticlayer 112 may also be beneath the pinned layer 110. A top view of thestructure 400 (in FIG. 7B) may show that the dielectric layer 108 isvisible. The fabrication method may include via etching into thedielectric layer 108, stopping on the pinned layer 110 to form thestructure 410 in the next step of the fabrication. From the side view, arecess 412 may be seen in the structure 410, and from the top view, thepinned layer 110 may be visible through the portion of the dielectriclayer 108 removed by etching. Via etching may produce a recess 412 invarious shapes within the structure 410, including the circular shape asdepicted in the top view of the structure 410. Nonmagnetic material maybe formed in the recess 412 to form a nonmagnetic spacer 114. Thenonmagnetic spacer 114 may be formed along the edge of the recess 412 sothat it is in contact with and perpendicular to the pinned layer 110, asseen from the top view of the structure 420 in FIG. 7B. After thenonmagnetic spacer 114 is formed along the edge of the recess 412, theremaining portion of the recess 412 may be filled with dielectricmaterial 108. The fabrication process may include planarizing by CMPsuch that the nonmagnetic spacer 114 in the resulting structure 430 maybe exposed at the top edge. As may be seen in the top view of thestructure 430, the nonmagnetic spacer 114, which is surrounded by thedielectric layer 108, may have a visible top edge.

After planarization, a free layer 106 may be deposited on the dielectriclayer 108 with the exposed nonmagnetic spacer 114, and an electrodelayer 102 may be disposed on the free layer 106. The resulting structure440 may be patterned such that the structure 440 may be isolated fromother structures or other memory cells. The top view of the structure440 in FIG. 7B depicts an example of how one magnetic cell structure 440may appear after patterning. While only the top electrode layer 102 maybe visible from the top view, this figure depicts an outline of a ring,which may represent the position of the underlying nonmagnetic spacer114.

Referring now to FIGS. 8A and 8B, the structure 450 may include adielectric layer 108 on top of a pinned layer 110. An antiferromagneticlayer 112 may also be beneath the pinned layer 110. A top view of thestructure 450 (in FIG. 8B) may show that the dielectric layer 108 isvisible. The fabrication method may include etching the dielectric layer108 into a mesa, and stopping on the pinned layer 110, as seen in thenext structure 460 of the fabrication process. As can be seen from thetop view of the structure 460, the etching may stop on the pinned layer110 so that it is visible around the mesa, or the remaining portion ofthe dielectric layer 108. The dielectric mesa 108 may be in any shape,including the round shape depicted in the top view of the structure 460.A nonmagnetic spacer 114 may then be formed along the side of thedielectric mesa 108, and may be perpendicular to the pinned layer 110.The top view of this structure 470 depicts the nonmagnetic spacer 114around the side of the dielectric mesa 108, which may resemble a ringaround a round dielectric mesa 108. The next step in the fabrication mayinvolve depositing dielectric material 108 to surround the nonmagneticspacer 114 of the structure 480, and a top view of the structure 480shows that the nonmagnetic spacer 114 may be surrounded by thedielectric layer 108 such that only the top edge of the nonmagneticspacer 114 is revealed. The free layer 106 is then deposited on thedielectric layer 108 and the nonmagnetic spacer 114. As the top edge ofthe nonmagnetic spacer 114 was exposed, the nonmagnetic spacer 114 mayserve as a bridge between the pinned layer 110 and the free layer 106.An electrode layer 102 may be disposed on the free layer 106. Thestructure 490 may be patterned to isolate the cell stack, as may be seenby the top view of the structure 490 in FIG. 8B.

In the embodiments of FIGS. 7A-8B, the cross sectional area of theprogramming current through the free layer 106 may be limited by thesmall cross sectional area of the current path immediately beforeentering the free layer 106. More specifically, the programming currentflows from the pinned layer 110 through the nonmagnetic spacer 114before reaching the free layer 106. The cross sectional area of theprogramming current is small immediately before reaching the free layer106, as the cross sectional area of the programming current in thenonmagnetic spacer 114 is limited to the area of the thin ring shape.Thus, the cross sectional area of the programming current may also besmall in the free layer 106 and may be approximated by the circumferenceof the nonmagnetic spacer 114 and the thickness of the nonmagneticspacer 114. A smaller programming current may then have a larger currentdensity in the portion of the free layer 106 through which the currenttravels than if the same programming current were traveling axiallythrough the entire cross sectional area of the free layer 106.

As previously explained, the programming current must reach some currentdensity in the free layer 106 such that the spin torque of the electronsin the programming current may switch the magnetization of the freelayer 106 to write the cell to a high or low resistance state. Using thestructure 490 in FIG. 8A as an example, if the programming current wasflowing up to the free layer 106, the current density may be sufficientto switch the magnetization of the free layer because the crosssectional area of the programming current through the free layer 106 issmaller. The spin torque of the downward flowing electrons would switchthe magnetization in the free layer 106 to the left, such that thepinned layer 110 and the free layer 106 are antiparallel, and thestructure 490 is programmed to a high resistance state.

Read operations may also involve sending a read current, which maytravel through the cell to determine the resistance between the freelayer 106 and the pinned layer 110. As the read current may also passthrough the nonmagnetic spacer 114, the read current may read theresistance of the structure across the portion of the free layer 106that has been switched by the programming current.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A method of operating a memory cell comprising:directing a programming current through a free layer in the memory cellsuch that a cross sectional area of the programming current through thefree layer is smaller than the cross section of the memory cell; andprogramming the memory cell.
 2. The method, as set forth in claim 1,wherein the cross sectional area of the programming current through thefree layer of the memory cell is smaller than the axial cross section ofthe free layer.
 3. The method, as set forth in claim 1, whereindirecting a programming current through the free layer comprisesdirecting the programming current to flow laterally in the free layer,wherein a lateral cross section of the free layer is smaller than anaxial cross section of the free layer.
 4. The method, as set forth inclaim 1, comprising: directing the programming current to flow through anonmagnetic bridge, wherein a cross section of the nonmagnetic bridge issmaller than a cross section of the memory cell.
 5. The method, as setforth in claim 4, wherein the programming current is directed from apinned layer in the memory cell, through the nonmagnetic bridge, andthrough the free layer.
 6. The method, as set forth in claim 4, whereinthe cross sectional area of the programming current through the freelayer is approximately the cross section of the nonmagnetic bridge. 7.The method, as set forth in claim 4, wherein the nonmagnetic bridge is anonmagnetic layer, a nonmagnetic spacer, or a nonmagnetic liner in thememory cell.
 8. The method, as set forth in claim 1, wherein the memorycell is programmed when a density of the programming current through thefree layer is high enough to switch a magnetization of the free layer tobe either parallel or antiparallel to a magnetization of a pinned layerin the memory cell.
 9. A method of operating a memory cell comprising:directing a programming current through a magnetic cell structurecomprising a pinned ferromagnetic layer, a free ferromagnetic layer andan intermediate layer formed therebetween, wherein the programmingcurrent is directed through the free ferromagnetic layer through anonmagnetic bridge having a cross sectional area that is smaller thanthe cross section of the magnetic cell structure; and programming thememory cell.
 10. The method, as set forth in claim 9, wherein directinga programming current through the magnetic cell structure comprisesdirecting the programming current through a magnetic tunnel junction(MJT).
 11. The method, as set forth in claim 9, wherein directing aprogramming current through the magnetic cell structure comprisesdirecting the programming current through a spin valve structure. 12.The method, as set forth in claim 9, wherein the nonmagnetic bridge isformed inside the intermediate layer.
 13. The method, as set forth inclaim 9, wherein the nonmagnetic bridge is surrounding the intermediatelayer.
 14. The method, as set forth in claim 9, wherein the nonmagneticbridge comprises Cu, Au, Ta, Ag, CuPt, CuMn, or any combination thereof15. The method, as set forth in claim 9, wherein the nonmagnetic bridgecomprises a conductive material.
 16. A method of operating a memory cellcomprising: directing a programming current between a free layer of thememory cell and a pinned layer of the memory cell through a nonmagneticbridge coupled to each of the free layer and the pinned layer, wherein across sectional area of the programming current through the nonmagneticbridge is smaller than a cross sectional area of the free layer; andprogramming the memory cell.
 17. The method, as set forth in claim 16,wherein the programming current is directed from a pinned layer in thememory cell, through the nonmagnetic bridge, and through the free layer.18. The method, as set forth in claim 16, wherein the cross sectionalarea of the programming current through the free layer is approximatelythe cross section of the nonmagnetic bridge.
 19. The method, as setforth in claim 16, wherein the nonmagnetic bridge is a nonmagneticlayer, a nonmagnetic spacer, or a nonmagnetic liner in the memory cell.20. The method, as set forth in claim 16, wherein the memory cell isprogrammed when a density of the programming current through the freelayer is high enough to switch a magnetization of the free layer to beeither parallel or antiparallel to a magnetization of a pinned layer inthe memory cell.